A Variety of circuit designs have been developed for DC to DC power conversion. Such circuits typically provide power from a DC source at one voltage level to a load at a controlled voltage or power level. Typical converter circuits use semiconductor devices, such as power transistors, silicon controlled rectifiers (SCRs), Gate-Turn-Off (GTO) devices, Insulated-Gate-Bipolar-Transistors (IGBT), and so forth. Of particular interest in the design of such circuits are the voltage and current control obtained at the output and the efficiency and cost of the circuit.
While conventional DC to DC converter circuits convert power from a voltage source, such circuits are not well suited to converting power from a current source, such as a superconducting electrical storage magnet. Examples of circuits for supplying power to and delivering power from superconducting storage magnets are shown in the patents to the first switch and the primary of the transformer. The secondary of the transformer is connected through a rectifier to an energy storage device, such as a large capacitor or battery, which is charged by the current from the secondary and which supplies a substantially constant or controllable output voltage to a load connected across the energy storage device. For maximum efficiency, it is preferred that a full-wave rectifier is utilized at the secondary of the transformer to rectify the current provided to the energy storage device.
In operation, the second or bypass switch, which is connected across the current source, is initially closed, so that the current from the current source circulates through the bypass switch. The first switch, connected in series with the primary of the transformer, is open. Upon initiation of a cycle, the first switch is closed and the second switch is opened, which causes the current in the primary and secondary to increase rapidly. A bypass resistor across the second switch controls the maximum voltage applied to the primary of the transformer. The voltage across the energy storage device in the secondary increases as current flows into it up to a maximum level and then declines again as the current in the secondary of the transformer declines to zero. When the voltage across the storage device reaches a selected minimum level, the second switch is closed, and the first switch is opened, causing the current in the primary to be shunted through a bypass resistor connected across the first switch and rapidly decline to zero. The current in the secondary rapidly reaches a maximum and then declines steadily. This results in an increase in the output voltage across the energy storage device as the current flows through it until it reaches a maximum, and then the output voltage declines as the current in the secondary declines and reaches zero. When the voltage across the storage device reaches the selected minimum level, the first switch is again closed and the second switch is again opened to initiate a new cycle. The cycle of alternately opening and closing the Peterson, et al., U.S. Pat. Nos. 4,079,305 and 4,122,512. An example of a circuit for supplying power unidirectionally from a current source to a voltage load is shown in U.S. Pat. No. 4,675,797 to Vinciarelli.
When supplying power from a superconducting energy storage magnet, a significant consideration is that the storage magnet often carries very large currents which must be switched. Since energy is lost in switching operations, particularly in semiconductor switches, conventional DC to DC converter circuits which utilize fast-switching semiconductor switches have significant inefficiencies in the switching system, as well as typically requiring numerous high cost switching components connected in parallel to adequately handle the large currents being switched.